A downhole casing in a well bore may have several leaks and/or weakened casing parts or other irregularities which must be sealed off to secure that no unintended fluid or gases enter the casing through the leaks or potential leaks in the weakened casing parts.
Also, the downhole casing may have perforations made for allowing crude oil or gases to enter the casing from the surrounding formation. However, it may also be necessary to seal off these perforations when the layer of oil has moved to ensure that no unintended fluid, such as water, enters the casing and mixes with the oil or gas.
In prior art solutions, the sealing of leaks, perforations and/or weakened casing parts, or other irregularities downhole in a casing is performed by introducing liners or patches into the casing, positioning the liners opposite the leaks, perforations and/or weakened casing parts, and subsequently expanding the liners.
Due to the present production technology, it is not possible to produce patches having a certain length while still maintaining their expansion ability without them breaking. However, when sealing off a perforation zone, the patches used are often not long enough, making it necessary to use several patches. Therefore, many attempts have been made to develop a solution where patch pieces overlap, as shown in FIG. 1. The prior art solution of FIG. 1 shows a leak in a casing, which has been sealed by means of two patch pieces. However, using two or more patch pieces with overlapping ends to obtain a secure sealing along the entire patch area has proven very difficult to handle downhole, as shown in FIGS. 2 and 3.
As shown in FIG. 2, when expanding the patch pieces, they may overlap too much, resulting in one of the patch pieces projecting inwards from the other patch piece, thereby reducing the inner diameter of the casing. This failure requires additional work to remove the projecting part to ensure that the inner diameter of the casing is not reduced any more than absolutely necessary. This takes time, and removing the projecting part may risk new leaks occurring.
To avoid the patch pieces overlapping too much, they may be placed with a smaller overlap, but the intended overlap between the patch pieces is often not large enough, resulting in an opening between the patches, which still creates a leak area, as shown in FIG. 3.